1. Field of the Invention
The present invention relates to a motion determining apparatus, a motion determining method, and a picture information converting apparatus.
2. Description of the Related Art
In a picture signal converting apparatus that converts an input digital picture signal into a picture signal with a different scanning line structure or in a picture process using a highly efficient code for compressing a digital picture signal, a motion determining process for determining whether or not a considered portion of a picture has a motion is used. In a conventional motion determining apparatus, the sum of the absolute values of differences between a picture portion (for example, a block) of one frame and the same picture portion of an adjacent frame. When the sum is equal to or larger than a predetermined threshold value, it is determined that the picture portion has a motion. In contrast, when the sum is smaller than the predetermined threshold value, it is determined that the picture portion does not have a motion.
However, in the conventional motion determining process, since it is determined whether a picture is a moving picture or a still picture, at the boundary of a picture portion that has been processed as a still picture and another picture portion that has been processed as a moving picture, there is a remarkable difference in the picture quality. In addition, when the motion determining process is performed with only a frame difference, an incorrect result may be obtained. In other words, even if a frame difference of a picture is large, if there is a large intra-field activity, the picture may be close to a still picture. In contrast, even if a frame difference of a picture is small, if there is no intra-field activity, the picture may be close to a moving picture. As a motion determining method using the fact that the relation between a frame difference and an intra-field activity is linear, slope method is known. However, in the slope method, the motion determining process cannot be flexibly performed corresponding to the value of the intra-field activity.